Project Abstract Streptococcus pneumoniae is one of the world's most debilitating human pathogens, and in the US is the most common cause of community acquired bacterial pneumoniae, despite having an available vaccine. As the vaccine can only contain some of the strains needed for immunity, we must find alternative treatments that target all 93+ different versions (or serotypes) of this deadly pathogen. One of the largest problems in pneumococcal research is understanding how this pathogen can quickly convert from a harmless nasal commensal to invasive pathogen. Key to understanding this crucial transition is the regulation of one of S. pneumoniae's most important virulence factors, the protective polysaccharide (sugar-based) capsule it uses to avoid our immune system. Our lab has now identified a crucial set of proteins that work together in S. pneumoniae, and based on their high conservation likely many other pathogens, to enable invasiveness and disease. We have named the collective proteins and small molecules that regulate this process the Capsule Regulatory Cascade, or CRC. In this work we will elucidate how the CRC enables S. pneumonaie to invade the lung by understanding the molecular and cellular interactions of this signaling network. Our methods to achieve this goal will include looking at the CRC at the smallest level (atoms), through visualization of its action within whole live animals in real time. Results from our studies will give unprecedented insight as to how this pathogen uses the CRC to cause disease, paving the way for novel therapeutic strategies targeting all serotypes rather than just the subset contained in the current vaccines. Finally, the CRC shares homology with proteins in humans that are crucial regulators of human disease, such as cancer, cardiomyopathy and epilepsy. We thus predict that our data will lend valuable insight into the mechanism by which these and other human diseases manifest.